The invention is based on a priority application EP 06290841.3 which is hereby incorporated by reference.
The invention relates to a method as a pre-condition for a gain control of an optical amplifier that amplifies a whole signal out of a multiple of optical signals and where each of these signals includes regular gaps.
The invention further relates to a tree-shaped optical access network with a central station and at least one optical access line connected thereto for a whole signal out of a multiple of optical signals destined for incoming at the central station, where each of these signals includes regular gaps and comes from different terminals and with a repeater looped into the optical access line and including an optical amplifier for the whole signal.
The invention further relates to a central station and to a terminal.
Such a tree-shaped optical access network is shown in FIG. 1 of “Fast Switching and Gain Control of Optical Repeater Units for Burst Mode Upstream Transmission Over Super-PONs” by J. Zhou et al. in Proceedings of the SPIE—The International Society for Optical Engineering (1996), vol. 2893, pp. 19-24.
Though such access networks include at least one optical amplifier, that is an active element, these networks commonly are called PONs, that is Passive Optical Networks.
At the branched ends of such networks a multiple of terminals is located that more or less independently send their information to the central station. When they do this with different wavelengths, they are at first glance really independent from one another.
Such terminals do not work in a continuous, synchronous way as with former transmission techniques, but they work in an asynchronous, bursty way. One aspect thereof is that the amplitude of the combined signals is not constant.
The document by J. Zhou et al. cited at the beginning works with Semiconductor Optical Amplifiers, SOAs, whose gains are largely insensitive to changing power levels below saturation. But another type of optical amplifiers, the Erbium Doped Fiber Amplifiers, EDFAs, is sensitive thereto. This means in particular, that varying signal strength on one wavelength can alter the gain experienced by a signal on a second wavelength. Fortunately such amplifiers have other characteristics that make them preferable anyway. That is why one has to cope with the unwanted characteristics mentioned first.
One aspect of the unwanted characteristics is a certain amount of gain coupling between the different single signals out of the combined whole signal. This cross gain effect mainly is the result of some kind of memory effect for slow signal components offered by those amplifiers.
What especially attracts attention is the situation of inserting a new terminal to the access network or restoring the network after an interruption. In such situation inter alia the precise gain control of the amplifier(s) is important to allow for the setting of a reliable detection threshold. Here cross gain effects are very much disturbing.